Patients will be enrolled at two OUTCOMES RESEARCH Group sites: the University of Louisville and the Cleveland Clinic. Patients undergoing gastric bypass will all be given 80% intraoperative oxygen and then randomly assigned to 30% or 80% oxygen until the first postoperative morning. The primary outcome will be a composite of major complications plausibly related to infection or healing. A maximum of 1,276 patients will provide a 90% power to detect a 25% reduction in the proportion of patients with at least one major complication. Supplemental oxygen is safe and inexpensive, costing only a few dollars per patient. Confirming our hypothesis would indicate that clinicians can reduce the risk of serious complications by making an essentially trivial modification in postoperative patient care.